Image pickup apparatus and image pickup system

ABSTRACT

An image pickup apparatus performs a global electronic shutter operation in which exposure periods of a plurality of pixels coincide with one another. In a first period in which a photoelectric conversion unit of at least one of the pixels stores charge, signals based on charges stored in holding units of the pixels are successively output to output lines. In a second period after the output of the signals from the pixels is terminated, the holding units of the pixels hold charge.

CROSS-REFERENCE TO RELATED APPLICATION

This is a Continuation of U.S. Ser. No. 15/387,487 filed Dec. 21, 2016which is a Continuation of U.S. application Ser. No. 14/645,233 filedMar. 11, 2015, now an issued patent U.S. Pat. No. 9,560,285 issue dateJan. 31, 2017, which claims priority from Japanese Patent ApplicationNo. 2014-052327, filed Mar. 14, 2014, which are hereby incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present disclosure relates to an image pickup apparatus and an imagepickup system.

Description of the Related Art

In recent years, employment of a global electronic shutter in CMOS imagesensors has been proposed. Image pickup apparatuses disclosed inJapanese Patent Laid-Open Nos. 2004-111590 and 2006-246450 have anadvantage in that, even when an image of a quickly moving object iscaptured, the object image is not distorted.

SUMMARY OF THE INVENTION

According to an aspect of the present disclosure, an image pickupapparatus includes a plurality of pixels. Each pixel includes aphotoelectric conversion unit configured to generate charge in responseto incident light and to accumulate the charge, a holding unitconfigured to hold the charge, an amplification unit configured tooutput a signal based on the charge, a first transfer switch configuredto transfer the charge from the photoelectric conversion unit to theholding unit, and a second transfer switch configured to transfer thecharge from the holding unit to the amplification unit. The image pickupapparatus includes an output line to which the signals from theplurality of pixels are output. At a first time point, the photoelectricconversion units of the plurality of pixels start accumulation ofcharge. The first transfer switch of at least one of the plurality ofpixels is kept off from the first time point to a second time point andthe photoelectric conversion unit of the at least one of the pluralityof pixels accumulates charge generated in a first period from the firsttime point to the second time point. In the first period, the secondtransfer switches of the plurality of pixels are turned on and theamplification units of the plurality of pixels output the signals to theoutput line. In a second period from the second point to a third point,the holding units of the plurality of pixels hold charge generated bythe photoelectric conversion units in the first period and chargegenerated by the photoelectric conversion units in the second period. Atthe third time point, the first transfer switches of the plurality ofpixels are controlled from on to off.

According to an aspect of the present disclosure, an image pickupapparatus includes a plurality of pixels. Each pixel includes aphotoelectric conversion unit configured to generate charge in responseto incident light and to accumulate the charge, a holding unitconfigured to hold the charge in a portion different from thephotoelectric conversion unit, and an amplification unit configured tooutput a signal based on the charge. The image pickup apparatus includesan output line to which the signals from the plurality of pixels areoutput. Charge generated in a first period is accumulated in thephotoelectric conversion units of the plurality of pixels. During asecond period following the first period, the holding units of theplurality of pixels hold charge generated by the photoelectricconversion units in the first period and charge generated by thephotoelectric conversion units in the second period. In each of theplurality of pixels, charge held by the holding units is read to theamplification units in the first period.

Further features of the present disclosure will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an equivalent circuit of an imagepickup apparatus.

FIG. 2 is a diagram schematically illustrating a cross-section structureof the image pickup apparatus.

FIG. 3 is a diagram illustrating driving pulses of the image pickupapparatus.

FIG. 4 is a diagram illustrating driving pulses of the image pickupapparatus.

FIG. 5 is a diagram schematically illustrating an operation of the imagepickup apparatus.

FIG. 6 is a diagram schematically illustrating a cross-section structureof the image pickup apparatus.

FIG. 7 is a diagram illustrating an equivalent circuit of an imagepickup apparatus.

FIG. 8 is a diagram schematically illustrating a cross-section structureof the image pickup apparatus.

FIGS. 9A and 9B are diagrams illustrating driving pulses of the imagepickup apparatus.

FIG. 10 is a diagram schematically illustrating a cross-sectionstructure of an image pickup apparatus.

FIG. 11 is a diagram schematically illustrating a cross-sectionstructure of an image pickup apparatus.

FIG. 12 is a diagram illustrating driving pulses of an image pickupapparatus.

FIG. 13 is a block diagram illustrating a configuration of an imagepickup system.

DESCRIPTION OF THE EMBODIMENTS

According to embodiments below, a global electronic shutter may beoperated while saturation charge quantity, or saturation charge amount,may be improved.

In an image pickup apparatus disclosed in Japanese Patent Laid-Open No.2004-111590, all charges generated by photoelectric conversion forobtaining one image or one frame are accumulated in photoelectricconversion units. Thereafter, the charges in all pixels aresimultaneously transferred from the photoelectric conversion units toholding units, and photoelectric conversion is started to obtain a nextimage or a next frame. Therefore, in order to increase saturation chargequantity of the pixel, saturation charge quantity of the photoelectricconversion unit and saturation charge quantity of the holding unit aresubstantially the same. When the saturation charge quantity of thephotoelectric conversion unit increases, an area of the photoelectricconversion unit increases. Therefore, a size of the pixel may increase.

In the image pickup apparatus disclosed in Japanese Patent Laid-Open No.2006-246450, the photoelectric conversion unit does not accumulatecharges but the holding unit stores almost all the charges. Therefore,saturation charge quantity of pixel may be increased without increasingsaturation charge quantity of the photoelectric conversion unit.However, a period in which generated charges are not allowed to bestored exists in this method, and therefore, image quality may bedegraded.

The inventers found that, in some image pickup apparatuses, it isdifficult to increase saturation charge quantity of pixels. According tosome embodiments described herein, in an image pickup apparatus whichemploy a global electronic shutter, saturation charge quantity of pixelsmay be increased.

An embodiment provides an image pickup apparatus including a pluralityof pixels and an output lines to which signals are supplied from theplurality of pixels. Each of the pixels includes a photoelectricconversion unit, a holding unit which holds charge, and an amplificationunit which outputs a signal based on the charge. Each of the pixelsfurther includes a first transfer switch which transfers charge from thephotoelectric conversion unit to the holding unit and a second transferswitch which transfers charge from the holding unit to the amplificationunit. With this configuration, an image pickup operation in whichphotoelectric conversion periods coincide, that is, the so-called“global electronic shutter”, may be operated. An electronic shutter is,for example, defined as to electrically control accumulation of chargethat has been generated in response to incident light. A transfer switch(or transistor) and/or a discharging switch (or transistor) may be usedto define the period of accumulation.

In embodiments of the present disclosure, photoelectric conversion unitsof pixels simultaneously start accumulation, or storage, of charge at afirst time point. A first transfer switch of at least one of the pixelsis kept being off from the first time point to, or until, a second timepoint. In the at least one of the pixels, charge generated in thisperiod of time is accumulated, or stored, in the photoelectricconversion unit. The period from the first time point to the second timepoint corresponds to a first period. In other words, the first periodmay be defined as starting at the first time and ending at the secondtime.

In the first period, the output units of the pixels output signals basedon charges held in holding units of the pixels to the output line inturns, or successively. In other words, each of the pixels outputs atleast one signal in the first period. Specifically, the first transferswitches of the pixels are turned on in the first period in turns. Sincethe charges generated in the first period are accumulated in thephotoelectric conversion units, the holding units may hold, in the firstperiod, charges generated before the first time point.

The number of signals output in the first period may be changeddepending on a format of an image to be output. In a case of shooting ofmoving images, for example, it is sufficient that a number of signalscorresponding to the number of horizontal lines used for one frame areoutput. In such an embodiment, it is not necessarily the case that allthe pixels included in the image pickup apparatus output signals.

After signals are output from the pixels, the holding units of thepixels hold, or store, charges at least in a second period from thesecond time point to a third time point. The second period may bedefined as starting at the second time and ending at the third time.During the second period, the holding units hold the charges generatedin the first period and the charges generated in the second period. Atthe third time point, the first transfer switches of the pixels aresimultaneously controlled from an on state to an off state. In otherwords, at the third time point, the first transfer switches of thepixels are simultaneously controlled from being on to being off.

Since the photoelectric conversion units at least store the chargesgenerated in the first period, even if saturation charge quantity of thephotoelectric conversion units are small, saturation charge quantity ofthe pixels may be maintained or even increased. Accordingly, with thisconfiguration, the global electronic shutter may be operated while thesaturation charge quantity may be maintained. Note that, in someembodiments, the second period in which the holding units of the pixelshold charges is longer than the first period. This is because, in a casethat the second period is longer than the first period, the saturationcharge quantity of the photoelectric conversion units may be reduced.

The embodiments of the present disclosure will be described hereinafterwith reference to the accompanying drawings. Embodiments of the presentdisclosure are not limited to the embodiments described below. Forexample, an embodiment in which a configuration of a portion of one ofthe embodiments below is added to one of the other embodiments and anembodiment in which a configuration of a portion of one of theembodiments below is replaced by a configuration of a portion of one ofthe other embodiments are also included in the embodiments of thepresent disclosure. Furthermore, in the embodiments below, a firstconductivity type is an N type and a second conductivity type is a Ptype. However, the first conductivity type may be a P type and thesecond conductivity type may be an N type.

First Embodiment

A first embodiment will be described. FIG. 1 is a diagram illustratingequivalent circuits of pixels of an image pickup apparatus. Althoughfour pixels 20 are illustrated in FIG. 1, the image pickup apparatusincludes more pixels.

Each of the pixels 20 includes a photoelectric conversion unit 1, aholding unit 2, an amplification unit 10, a first transfer switch 4, anda second transfer switch 5. Each of the pixels 20 further includes areset transistor 9 and a selection transistor 7.

The photoelectric conversion unit 1 generates charge in response toincident light. The photoelectric conversion unit 1 accumulates, orstores, charge generated in response to incident light. The firsttransfer switch 4 transfers the charge of the photoelectric conversionunit 1 to the holding unit 2. The holding unit 2 holds the chargegenerated by the incident light in a portion other than thephotoelectric conversion unit 1. The second transfer switch 5 transfersthe charge of the holding unit 2 to an input node 3 of the amplificationunit 10. The reset transistor 9 resets a voltage of the input node 3 ofthe amplification unit 10. The selection transistor 7 selects acorresponding one of the pixels 20 which outputs a signal to an outputline 8. The amplification unit 10 outputs a signal based on the chargegenerated by the incident light to the output line 8. The amplificationunit 10 is a source follower, for example. Furthermore, the firsttransfer switch 4 and the second transfer switch 5 are MOS transistors.

A control line Tx1 is connected to the first transfer switch 4. Acontrol line Tx2 is connected to the second transfer switch 5. In thisembodiment, a plurality of pixels are arranged in a matrix. A commoncontrol line is connected to pixels in the same row. Therefore, acontrol line Tx1(n) is connected to pixels in an n-th row.

With this configuration, charges generated while the holding units 2hold charges may be stored in the photoelectric conversion units 1.Accordingly, an image pickup operation in which periods of time in whichphotoelectric conversion is performed in the pixels match one another,that is, the so-called “global electronic shutter” may be operated.

FIG. 2 is a diagram schematically illustrating a cross-section structureof the image pickup apparatus. In FIG. 2, a cross section of one of thepixels 20 is illustrated. Portions having functions the same as those ofthe portions in FIG. 1 are denoted by reference numerals the same asthose of FIG. 1. Although a surface irradiation type image pickupapparatus is illustrated in FIG. 2, a back surface irradiation typeimage pickup apparatus may be employed.

The photoelectric conversion unit 1 has an embedded photodiodestructure. The photoelectric conversion unit 1 includes an N-typesemiconductor region 11 and a P-type semiconductor region 12. The N-typesemiconductor region 11 and the P-type semiconductor region 12 form a PNjunction. The P-type semiconductor region 12 enables suppression ofnoise of an interface.

A P-type semiconductor region 14 is a well. An N-type semiconductorregion 13 is disposed below the N-type semiconductor region 11. Impurityconcentration of the N-type semiconductor region 13 is lower than thatof the N-type semiconductor region 11. Accordingly, charge generated ina deep position is collected in the N-type semiconductor region 11.Here, the N-type semiconductor region 13 may be a P-type semiconductorregion 13. A P-type semiconductor region 17 serving as a potentialbarrier relative to charge is disposed below the N-type semiconductorregion 13.

The holding unit 2 includes an N-type semiconductor region 201. TheN-type semiconductor region 201 holds charge corresponding to a signal.Impurity concentration of the N-type semiconductor region 201 is higherthan that of the N-type semiconductor region 11.

A gate electrode 40 constitutes a gate of the first transfer switch 4.Furthermore, a gate electrode 50 constitutes a gate of the secondtransfer switch 5. A portion of the gate electrode 40 overlaps with theN-type semiconductor region 201 through a gate insulation film. A holemay be induced on a surface of the N-type semiconductor region 201 byapplying a negative voltage to the gate electrode 40. By this, noisegenerated in an interface may be suppressed.

Light to the holding unit 2 is blocked by a light shielding unit 203.The light shielding unit 203 is formed by metal, such as tungsten oraluminum, through which visible light is difficult to pass. A colorfilter 100 and a micro lens 101 are disposed on an opening of the lightshielding unit 203.

The photoelectric conversion unit 1 and the holding unit 2 are disposedon a semiconductor substrate. In this embodiment, an area of orthogonalprojection of the photoelectric conversion unit 1 to a surface which isparallel to a surface of the semiconductor substrate is smaller than anarea of orthogonal projection of the holding unit 2 to the same surface.With this configuration, the saturation charge quantity of the pixelsmay be effectively increased while reduction of noise is realized.

To increase the saturation charge quantity of the pixels, the holdingunits 2 preferably have a large saturation charge quantity. When theimpurity concentration of the N-type semiconductor region 201 in theholding unit 2 is increased or the area of the N-type semiconductorregion 201 is increased in a plan view, the saturation charge amount ofthe holding unit 2 may be increased. However, when the impurityconcentration of the N-type semiconductor region 201 is high, leakagecurrent or the like is likely to be large, and therefore, noise may beincreased. Therefore, the saturation charge amount may be increased byincreasing the area of the N-type semiconductor region 201 in a planview while the impurity concentration of the N-type semiconductor region201 is suppressed.

As described above, when the areas of the holding units 2 in a planview, that is, the areas of the orthogonal projection of the holdingunits 2 are increased, the saturation charge quantity of the pixels maybe increased while noise is reduced. In this case, areas of thephotoelectric conversion units 1 are relatively likely to be small in aplan view, and it is difficult to increase the saturation chargequantity of the photoelectric conversion units 1. Accordingly, eventhough the saturation charge quantity of the photoelectric conversionunits 1 are small, the saturation charge quantity of the pixels are moreeffectively maintained.

A method for driving the image pickup apparatus of this embodiment willbe described. FIG. 3 is a diagram schematically illustrating drivingpulses according to this embodiment. In FIG. 3, driving pulses suppliedto the control lines Tx1 of the first transfer switches 4 and thecontrol lines Tx2 of the second transfer switches 5 of the pixels in then-th row to an (n+2)-th row are illustrated. When a driving pulse is ina high level, a corresponding transistor or a corresponding switch isturned on. When a driving pulse is in a low level, a correspondingtransistor or a corresponding switch is turned off. These driving pulsesare supplied by a control unit included in the image pickup apparatus. Alogic circuit, such as a shift register or an address decoder, is usedas the control unit.

First, a preceding frame is exposed before a time point T1. The term“exposure” means that charge generated by photoelectric conversion isstored or held as a signal. Charges generated before the time point T1are held by the holding units 2. When the first transfer switches 4 forthe charges supplied from the photoelectric conversion units 1 to theholding units 2 of all the pixels are simultaneously turned off, theexposure of the preceding frame is terminated (at the time point T1 ofFIG. 1).

Furthermore, at the time point T1, all charges of the photoelectricconversion units 1 are transferred to the holding units 2. That is, thephotoelectric conversion units 1 enter an initial state. Therefore, atthe time point T1, the photoelectric conversion units 1 of the pixels inthe three rows simultaneously start storage of charges. In this way, thestorage of the charges by the photoelectric conversion units 1 isstarted when the first transfer switches 4 are turned off in thisembodiment.

In a first period from the time point T1 to a time point T2, off statesof the first transfer switches 4 are maintained. In this embodiment, offstates of the first transfer switches 4 of all the pixels aremaintained. However, it is sufficient that an off state of the firsttransfer switch 4 of at least one of the pixels is maintained in theperiod from the time point T1 to the time point T2.

The time point T2 is reached when the first period is elapsed after thetime point T1. Specifically, the first period corresponds to the periodfrom the time point T1 to the time point T2. In the first period, thecharges generated in the first period are stored in the photoelectricconversion units 1. Meanwhile, the holding units 2 hold the chargesgenerated in the preceding frame in the first period.

In the first period, the charges in the holding units 2 are sequentiallyread to the input nodes 3 of the amplification units 10. Specifically,when the second transfer switches 5 in the n-th row are turned on, thecharges of the holding units 2 of the pixels in the n-th row aretransferred to the input nodes 3. Voltages of the input nodes 3 arechanged in accordance with capacitances of the input nodes 3 and amountsof the transferred charges. The amplification units 10 output signalsbased on the voltages of the input nodes 3 to the output lines 8. Next,the same operation is performed on pixels in the (n+1)-th row. Thisoperation is performed on pixels from a first row to pixels in a lastrow. After the reading is performed on a last pixel, the first transferswitches 4 and the second transfer switches 5 of all the pixels are inan off state.

At the time point T2, the first transfer switches 4 are turned on. Bythis, the charges of the photoelectric conversion units 1 aretransferred to the holding units 2. Specifically, the charges generatedin the first period are held by the holding unit 2 after the time pointT2. In this embodiment, the off states of the first transfer switches 4of all the pixels are simultaneously changed to on states. However, itis sufficient that the first transfer switches 4 of the pixels are in onstates by the time point T2, and timings of the change may be shiftedfrom one another. For example, the first transfer switches 4 of thepixels which have been subjected to the reading operation describedabove may be successively turned on.

Thereafter, in a second period from the time point T2 to a time pointT3, the holding units 2 hold the charges generated in the first periodand charges generated in the second period. In this embodiment, the onstates of the first transfer switches 4 are maintained in the secondperiod. Therefore, the charges generated in the second period areimmediately transferred to the holding units 2. Note that a period oftime in which the charges are transferred from the photoelectricconversion units 1 to the holding units 2 may be arbitrarily set. In aportion of the second period, the first transfer switches 4 may be in anoff state.

At the time point T3, the on states of the first transfer switches 4 ofthe pixels in all the rows are simultaneously changed to off states. Bythis, an exposure period for one frame is terminated. As describedabove, exposure periods of all the pixels match one another.Specifically, in all the pixels, exposure is started at the time pointT1 and terminated at the time point T3. Furthermore, exposure of a nextframe is started at the time point T3, and thereafter, the operationfrom the time point T1 to the time point T3 is repeatedly performed.

Next, the operation of reading a signal from one pixel will be brieflydescribed. FIG. 4 is a diagram schematically illustrating driving pulsesused in the image pickup apparatus. In FIG. 4, a driving pulse SEL to besupplied to the selection transistor 7, a driving pulse RES to besupplied to the reset transistor 9, and a driving pulse TX2 to besupplied to the second transfer switch 5 are illustrated. When a drivingpulse is in a high level, a corresponding transistor or a correspondingswitch is turned on. When a driving pulse is in a low level, acorresponding transistor or a corresponding switch is turned off.

In accordance with the driving pulses illustrated in FIG. 4, selectionof pixels, resetting, reading of a noise signal (N read), transfer ofcharges, and reading of an optical signal (S read) are performed. Anoutput signal may be subjected to AD conversion in an outside of theimage pickup apparatus. The AD conversion may be performed in an insideof the image pickup apparatus.

Next, an effect of this embodiment will be described. FIG. 5 is adiagram schematically illustrating an operation of the image pickupapparatus. In FIG. 5, an image pickup operation performed on the n-thframe to the (n+1)-th frame is illustrated. An operation performed onthe n-th frame is denoted by a solid line and an operation performed onthe (n+1)-th frame is denoted by a dotted line.

In FIG. 5, exposure periods of the frames, periods of time in which thephotoelectric conversion units 1 store charges, and periods of time inwhich the holding units 2 hold charges are illustrated. According toFIG. 5, a reading operation is performed on a plurality of pixels in thefirst period. The reading operation in FIG. 5 includes transfer ofcharges using the second transfer switches 5 and output of signalsperformed by the amplification units 10 described with reference toFIGS. 3 and 4.

As illustrated in FIG. 5, immediately after exposure of one frame isterminated, next exposure may be started. By this, a period of time inwhich information lacks is substantially eliminated, and accordingly,image quality may be improved.

Furthermore, as illustrated in FIG. 5, the reading operation isindividually performed on the pixels during the first period in whichthe photoelectric conversion units 1 store the charges. Accordingly,even when saturation charge quantity of the photoelectric conversionunits 1 is small, saturation charge quantity of the pixels may beincreased. The saturation charge quantity of the pixel corresponds tothe maximum value of charge amounts that is used as a signal in chargesgenerated in one exposure, or a single frame. The saturation chargequantity of the photoelectric conversion units 1 corresponds to themaximum value of amounts of charges allowed to be stored in thephotoelectric conversion units 1. The saturation charge quantity of theholding units 2 corresponds to the maximum value of amounts of chargesallowed to be stored in the holding units 2.

One exposure period is obtained as a sum of the first period and thesecond period. Here, charges of a preceding frame held in the holdingunits 2 are read in the first period. Therefore, after the first period,the holding units 2 may hold charges. Accordingly, it is sufficient thatthe photoelectric conversion units 1 at least store charges generated inthe first period. Normally, amounts of charges generated in the firstperiod are smaller than those generated in one exposure period, andaccordingly, the saturation charge quantity of the photoelectricconversion units 1 may be reduced.

As illustrated in FIG. 5, in this embodiment, the second period in whichthe holding units 2 hold the charges is longer than the first period.Therefore, the saturation charge quantity of the photoelectricconversion units 1 may be further reduced. However, the first period maybe equal to the second period, or the first period may be longer thanthe second period.

In FIG. 5, a case where the reading operation is successively performedfrom the first row is illustrated as an example. However, order of thereading operation is not limited to this example. The reading operationis performed at least once on each of the pixels included in one framein the first period. Furthermore, in at least some of the pixels, aperiod of time from when the holding units 2 start holding of charges ina certain frame to when the holding units 2 start holding of charges ina next frame is equal to the exposure time.

It is preferable that a ratio of a sum of the first period and thesecond period to the first period is substantially the same as a ratioof a saturation charge quantity of the holding unit 2 to a saturationcharge amount of the photoelectric conversion unit 1. Here, the sum ofthe first period and the second period corresponds to one exposureperiod.

In this embodiment, a ratio of one exposure period to the first periodis 4:1. That is, the first period corresponds to a quarter of oneexposure period. In a case where a moving image of 60 frames per secondis to be captured, for example, the first period corresponds to 1/240seconds.

Therefore, it is preferable that a ratio of a saturation charge quantityof the holding unit 2 to a saturation charge quantity of thephotoelectric conversion unit 1 is close to 4:1. This is because,although the holding unit 2 holds all charge generated in one exposureperiod, the photoelectric conversion unit 1 holds at least a quarter ofthe charge. This ratio of the saturation charge quantity enablesoptimization of sizes of the photoelectric conversion unit 1 and theholding unit 2.

Note that the image pickup apparatus of this embodiment may have anoperation mode for performing rolling shutter. In the operation mode forthe rolling shutter, the photoelectric conversion units 1 of the pixelssuccessively start storage of charges. Thereafter, the first transferswitches 4 of the pixels are successively turned on. The image pickupapparatus of this embodiment may further have an operation mode forglobal electronic shutter employing another method. Examples of theglobal electronic shutter employing another method include an operationin which a period of time in which the photoelectric conversion units 1store charges becomes equal to the exposure period.

As described above, according to the image pickup apparatus of thisembodiment, the global electronic shutter may be operated while thesaturation charge quantity is increased.

Second Embodiment

A second embodiment will be described. In this embodiment, aconfiguration of holding units is different from that of the firstembodiment. Therefore, only portions different from those of the firstembodiment are described and descriptions of portions the same as thoseof the first embodiment are omitted.

An equivalent circuit of this embodiment is the same as that of thefirst embodiment.

Specifically, FIG. 1 is a diagram illustrating equivalent circuits ofpixels of an image pickup apparatus of this embodiment. Since adescription of FIG. 1 is the same as that of the first embodiment, thedescription is omitted here.

A driving method of this embodiment is the same as that of the firstembodiment. Specifically, FIGS. 3 and 4 are diagrams schematicallyillustrating driving pulses according to this embodiment. Furthermore,FIG. 5 is a diagram schematically illustrating an operation of the imagepickup apparatus of this embodiment. Since descriptions of FIGS. 3 to 5are the same as those of the first embodiment, the descriptions areomitted here.

FIG. 6 is a diagram schematically illustrating a cross-section structureof the image pickup apparatus. In FIG. 6, a cross section of a pixel isillustrated. Portions having functions the same as those of FIGS. 1 to 5are denoted by reference numerals the same as those in FIGS. 1 to 5.

A holding unit 2 includes an N-type semiconductor region 201 and aP-type semiconductor region 202. The P-type semiconductor region 202 isdisposed on the N-type semiconductor region 201. The P-typesemiconductor region 202 enables suppression of noise of an interface.

Furthermore, a gate electrode 40 of a first transfer switch 4 does notextend on the N-type semiconductor region 201. Therefore, restriction oflayout is eased, and a degree of freedom of design may be enhanced.

As described above, according to this embodiment, in addition to theeffect of the first embodiment, noise may be reduced.

Third Embodiment

A third embodiment will be described. This embodiment is different fromthe first and second embodiments in that pixels have discharge switches.Therefore, only portions different from those of the first and secondembodiments are described and descriptions of portions the same as thoseof the first and second embodiments are omitted.

FIG. 7 is a diagram illustrating equivalent circuits of pixels of animage pickup apparatus. Portions the same as those of FIG. 1 are denotedby reference numerals the same as those of FIG. 1. Note that, forsimplicity of the drawings, reference numerals of control lines Tx1 andTx2 are omitted. The control lines Tx1 and Tx2 have configurations thesame as those of the first embodiment.

Each of pixels has a discharge switch 18. The discharge switch 18discharges charge of a photoelectric conversion unit 1 to a power sourcenode, such as an overflow drain. A control line OFG is connected to thedischarge switch 18. The discharge switch 18 is a MOS transistor, forexample.

In the first embodiment, storage of charge in the photoelectricconversion unit 1 is started by changing a state of a second transferswitch 5 from an on state to an off state. In this embodiment, asillustrated in FIGS. 9A and 9B, start of exposure may be controlled bycontrolling the discharge switch 18. Specifically, storage of charge inthe photoelectric conversion unit 1 is started by changing a state ofthe discharge switch 18 from an on state to an off state. By this, anexposure period may be arbitrarily set.

FIG. 8 is a diagram schematically illustrating a cross-section structureof the image pickup apparatus. Portions having functions the same asthose of FIGS. 1 and 2 are denoted by reference numerals the same asthose in FIGS. 1 and 2. FIG. 8 is a diagram illustrating a case where,as with the second embodiment, a holding unit 2 includes a P-typesemiconductor region 202 as an example. The holding unit 2 may notinclude the P-type semiconductor region 202 as illustrated in FIG. 1.

The discharge switch 18 includes an overflow control electrode 16 and anoverflow drain 15. Charge of a photoelectric conversion unit 1 isdischarged to the overflow drain 15 in accordance with a voltagesupplied to the overflow control electrode 16. A predetermined voltageis supplied to the overflow drain 15. Light to the overflow controlelectrode 16 and the overflow drain 15 is blocked by the light shieldingunit 203.

A method for driving the image pickup apparatus of this embodiment willbe described. FIGS. 9A and 9B are diagrams schematically illustratingdriving pulses according to this embodiment. In FIGS. 9A and 9B, drivingpulses supplied to control lines Tx1 and Tx2 and the control line OFG inan n-th row to a (n+2)-th row are illustrated. The driving pulsessupplied to the control lines Tx1 and Tx2 are the same as those of thefirst embodiment.

When a driving pulse is in a high level, a corresponding transistor or acorresponding switch is turned on. When a driving pulse is in a lowlevel, a corresponding transistor or a corresponding switch is turnedoff. These driving pulses are supplied by a control unit included in theimage pickup apparatus. A logic circuit, such as a shift register or anaddress decoder, is used as the control unit.

Timings when the discharge switch 18 is operated in FIGS. 9A and 9B aredifferent from each other. In FIG. 9A, a state of the discharge switch18 is changed from an on state to an off state at a time point T4. Whilethe discharge switch 18 is in an on state, generated charge isdischarged. Therefore, according to driving illustrated in FIG. 9A, anexposure period corresponds to a period from the time point T4 to a timepoint T3. In FIG. 9B, a state of the discharge switch 18 is changed froman on state to an off state at a time point T5. Therefore, according todriving illustrated in FIG. 9B, an exposure period corresponds to aperiod from the time point T5 to the time point T3.

According to this embodiment, a driving method may be changed inaccordance with brightness of an object. For example, the driving pulsesillustrated in FIG. 3 are used in a normal state, the driving pulsesillustrated in FIG. 9A are used in a case of high brightness, and thedriving pulses illustrated in FIG. 9B are used in a case of higherbrightness.

Note that, in FIG. 9A, storage of charge in the photoelectric conversionunit 1 is started at the time point T4. Thereafter, in a period from thetime point T4 to the time point T3, an off state of the discharge switch18 is maintained. A reading operation is performed in accordance withthe driving pulses illustrated in FIG. 4.

According to this embodiment, in addition to the effect of the firstembodiment, the exposure period may be arbitrarily set.

Fourth Embodiment

A fourth embodiment will be described. This embodiment is different fromthe first to third embodiments in that a waveguide which guides light toa photoelectric conversion unit is provided. Therefore, only portionsdifferent from those of the first to third embodiments are described anddescriptions of portions the same as those of one of the first to thirdembodiments are omitted.

An equivalent circuit of this embodiment is the same as those of thefirst embodiment or the third embodiment. Specifically, FIGS. 1 and 7are diagrams illustrating equivalent circuits of pixels of an imagepickup apparatus of this embodiment. Since descriptions of FIGS. 1 and 7are the same as those of the first and third embodiments, thedescriptions are omitted here.

A driving method of this embodiment is the same as that of the firstembodiment or the third embodiment. Specifically, when a dischargeswitch is not provided, the driving pulses illustrated in FIGS. 3 and 4are used. When each of pixels has a discharge switch, the driving pulsesillustrated in FIGS. 9A, 9B, and 4 are used. Furthermore, FIG. 5 is adiagram schematically illustrating an operation of the image pickupapparatus of this embodiment. Since descriptions of FIGS. 3 to 5 andFIGS. 9A and 9B are the same as those of the first and thirdembodiments, the descriptions are omitted here.

FIG. 10 is a diagram schematically illustrating a cross-sectionstructure of the image pickup apparatus. Portions the same as those ofFIGS. 1, 2, 6, 7, and 8 are denoted by reference numerals the same asthose of FIGS. 1, 2, 6, 7, and 8. In FIG. 10, a case where a holdingunit 2 includes a P-type semiconductor region 202 as with the secondembodiment and a pixel includes a discharge switch 18 as with the thirdembodiment is illustrated as an example. However, the P-typesemiconductor region 202 and the discharge switch 18 may be omitted.

In this embodiment, a waveguide 301 is provided so as to correspond to aphotoelectric conversion unit 1. The waveguide 301 guides incident lightto the photoelectric conversion unit 1. By this, sensitivity may beimproved. In particular, degradation of sensitivity of light which isobliquely incident may be reduced.

The waveguide 301 has a general structure. In this embodiment, thewaveguide 301 is formed by a material having a refractive index higherthan that of a surrounding insulation film. An interlayer insulationfilm formed by a silicon oxide film is used as the surroundinginsulation film, and a silicon nitride film is used as the waveguide301, for example. Alternatively, a reflection layer is provided so as tosurround the waveguide 301. The waveguides 301 may be disposed for theindividual photoelectric conversion units 1 of all pixels or only forphotoelectric conversion units 1 of some of the pixels.

An interlayer lens 302 may be disposed between a color filter 100 andthe waveguide 301. The interlayer lens 302 collects light which passesthrough the color filter 100 in the waveguide 301. Use of the interlayerlens 302 may improve sensitivity. In particular, degradation ofsensitivity of light which is obliquely incident may be reduced.

As described above, according to this embodiment, in addition to theeffect of the first embodiment, sensitivity may be improved. Inparticular, the effect of improvement of the sensitivity is remarkablewhen an area of the photoelectric conversion unit 1 is reduced in a planview so that an area of the holding unit 2 is increased in the planview.

Fifth Embodiment

A fifth embodiment will be described. In this embodiment, aconfiguration of a holding unit is different from those of the first tofourth embodiments. Therefore, only portions different from those of thefirst to fourth embodiments are described and descriptions of portionsthe same as those of one of the first to fourth embodiments are omitted.

An equivalent circuit of this embodiment is the same as that of thefirst embodiment or the third embodiment. Specifically, FIGS. 1 and 7are diagrams illustrating equivalent circuits of pixels of an imagepickup apparatus of this embodiment. Since descriptions of FIGS. 1 and 7are the same as those of the first and third embodiments, thedescriptions are omitted here.

A driving method of this embodiment is the same as those of the firstembodiment or the third embodiment. Specifically, when a dischargeswitch is not provided, the driving pulses illustrated in FIGS. 3 and 4are used. On the other hand, when a discharge switch is provided, thedriving pulses illustrated in FIGS. 9A, 9B, and 4 are used. Furthermore,FIG. 5 is a diagram schematically illustrating an operation of the imagepickup apparatus of this embodiment. Since descriptions of FIGS. 3 to 5and FIGS. 9A and 9B are the same as those of the first and thirdembodiments, the descriptions are omitted here.

FIG. 11 is a diagram schematically illustrating a cross-sectionstructure of the image pickup apparatus. Portions the same as those ofFIGS. 1, 2, 6, 7, 8, and 10 are denoted by reference numerals the sameas those of FIGS. 1, 2, 6, 7, 8, and 10. In FIG. 11, a case where aholding unit 2 includes a P-type semiconductor region 202 as with thesecond embodiment and a pixel includes a discharge switch 18 as with thethird embodiment is illustrated as an example. However, the P-typesemiconductor region 202 and the discharge switch 18 may be omitted.Furthermore, in FIG. 11, a case where a waveguide 301 and an interlayerlens 302 are disposed is illustrated as an example. However, thewaveguide 301 and the interlayer lens 302 may be omitted.

In this embodiment, a P-type semiconductor region 303 and a P-typesemiconductor region 304 are disposed below the N-type semiconductorregion 201 which is included in the holding unit 2 and which holdscharge. The P-type semiconductor region 304 is disposed below the P-typesemiconductor region 303. Impurity concentration of the P-typesemiconductor region 303 is higher than that of the P-type semiconductorregion 304. With this configuration, charge of a deep portion of asubstrate is prevented from intruding into the N-type semiconductorregion 201. As a result, noise may be reduced.

Furthermore, in this embodiment, the P-type semiconductor region 304extends to a P-type semiconductor region 17. With this configuration,color mixture of charges of pixels may be reduced.

As described above, according to this embodiment, in addition to theeffect of the first embodiment, noise may be reduced.

Sixth Embodiment

A sixth embodiment will be described. A driving method of thisembodiment is different from those of the first to fifth embodiments.Therefore, only portions different from those of the first to fifthembodiments are described and descriptions of portions the same as thoseof one of the first to fifth embodiments are omitted.

An equivalent circuit of this embodiment is the same as that of thefirst embodiment or the third embodiment. Specifically, FIGS. 1 and 7are diagrams illustrating equivalent circuits of pixels of an imagepickup apparatus of this embodiment. Since descriptions of FIGS. 1 and 7are the same as those of the first and third embodiments, thedescriptions are omitted here.

A cross-section structure of a pixel of this embodiment is the same asthose of the first to fifth embodiments. Specifically, FIGS. 2, 6, 8,10, and 11 are diagrams schematically illustrating a cross-sectionstructure of a pixel of this embodiment.

A method for driving the image pickup apparatus of this embodiment willbe described. FIG. 12 is a diagram schematically illustrating drivingpulses according to this embodiment. In FIG. 12, driving pulses suppliedto control lines Tx1 and Tx2 and a control line OFG in an n-th row to a(n+2)-th row are illustrated. The driving pulses supplied to the controllines Tx1 and Tx2 and the control line OFG are the same as those of thefirst embodiment or the third embodiment. Note that, when the pixel doesnot include a discharge switch 18, a driving pulse is not supplied tothe control line OFG.

When a driving pulse is in a high level, a corresponding transistor or acorresponding switch is turned on. When a driving pulse is in a lowlevel, a corresponding transistor or a corresponding switch is turnedoff. These driving pulses are supplied by a control unit included in theimage pickup apparatus. A logic circuit, such as a shift register or anaddress decoder, is used as the control unit.

In this embodiment, a first transfer switch 4 is turned off in a portionof a second period. Specifically, at a time point T6, an on state of thefirst transfer switch 4 is changed to an off state. Thereafter, at atime point T7, the off state of the first transfer switch 4 is changedto an on state. With this configuration, a period of time in which thefirst transfer switch 4 is in an on state may be reduced. Consequently,noise generated by the first transfer switch 4 may be reduced.

In this embodiment, the off state of the first transfer switch 4 ischanged to an on state again at a time point T8. In this way, on/offcontrol of the first transfer switch 4 is performed a plurality of timesin the second period. With this configuration, the noise may be furtherreduced.

Furthermore, the number of times an off state is changed to an on stateis preferably equal to or larger than a ratio of a saturation chargeamount of a holding unit 2 to a saturation charge amount of aphotoelectric conversion unit 1. In this embodiment, the ratio of thesaturation charge amount of the holding unit 2 to the saturation chargeamount of the photoelectric conversion unit 1 is 4:1. Therefore, on/offcontrol of the first transfer switch 4 is performed four times in thesecond period.

As described above, according to this embodiment, in addition to theeffect of the first embodiment, noise may be reduced.

Seventh Embodiment

An embodiment of an image pickup system according to the presentinvention will be described. Examples of the image pickup system includea digital steel camera, a digital camcorder, a copier, a facsimile, acellular phone, an on-vehicle camera, and an observatory. Furthermore, acamera module including an optical system, such as a lens, and an imagepickup apparatus is also included in the image pickup system. FIG. 13 isa block diagram illustrating a digital still camera serving as anexample of the image pickup system.

In FIG. 13, a barrier 1001 protects a lens 1002, the lens 1002 forms anoptical image of an object on an image pickup apparatus 1004, and anaperture 1003 changes quantity of light which has passed the lens 1002.The image pickup apparatus described in the foregoing embodiments isdenoted by a reference numeral 1004, and the image pickup apparatus 1004converts an optical image formed by the lens 1002 into image data. Here,it is assumed that an AD conversion unit is formed on a semiconductorsubstrate of the image pickup apparatus 1004. A signal processing unit1007 performs various types of correction on image pickup data outputfrom the image pickup apparatus 1004 and compresses the image pickupdata. In FIG. 13, a timing generation unit 1008 outputs various timingsignals to the image pickup apparatus 1004 and the image processing unit1007, and an overall control/calculation unit 1009 controls the entiredigital still camera. A frame memory unit 1010 temporarily stores imagedata, an interface unit 1011 performs recording or reading on arecording medium, and a detachable recording medium 1012 is asemiconductor memory or the like for recording or reading image pickupdata. An interface unit 1013 is used to communicate with an externalcomputer and the like. Here, a timing signal or the like may be inputfrom an outside of the image pickup system which at least includes theimage pickup apparatus 1004 and the signal processing unit 1007 whichprocesses an image pickup signal output from the image pickup apparatus1004.

In this embodiment, a structure in which the image pickup apparatus 1004and an AD conversion unit are disposed on the same semiconductorsubstrate is described. However, the image pickup apparatus 1004 and theAD conversion unit may be disposed on different semiconductorsubstrates. Furthermore, the image pickup apparatus 1004 and the signalprocessing unit 1007 may be formed on the same semiconductor substrate.

While the present disclosure has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

What is claimed is:
 1. An image pickup apparatus comprising: a pluralityof pixels, each pixel including a photoelectric conversion unitconfigured to generate charge in response to incident light and toaccumulate the charge, a holding unit configured to hold the charge, anamplification unit configured to output a signal based on the charge, afirst transfer switch configured to transfer the charge from thephotoelectric conversion unit to the holding unit, and a second transferswitch configured to transfer the charge from the holding unit to theamplification unit; and an output line to which the plurality of pixelsare connected, wherein the image pickup apparatus performs a firstexposure having a first exposure period and a second exposure having asecond exposure period shorter than the first exposure period, the firstexposure period starts at a first time point, at the first time point,the photoelectric conversion units of the plurality of pixels startaccumulation of charge, the first transfer switch of at least one of theplurality of pixels is kept off from the first time point to a secondtime point and the photoelectric conversion unit of the at least one ofthe plurality of pixels accumulates charge generated in a first periodfrom the first time point to the second time point, in the first period,the second transfer switches of the plurality of pixels are turned onand the amplification units of the plurality of pixels output thesignals to the output line in turns, the first exposure period includesa second period after the first period, in the second period, theholding units of the plurality of pixels hold charge generated by thephotoelectric conversion units in the first period.
 2. The image pickupapparatus according to claim 1, wherein through a beginning of thesecond exposure period to an end of the second exposure period, a statewhere the first transfer switch is in an on state only comes once foreach of the plurality of pixels.
 3. The image pickup apparatus accordingto claim 1, wherein in the second exposure period, an output of thesignal by the amplification units of the plurality of pixels is notperformed.
 4. The image pickup apparatus according to claim 2, whereinin at least a portion of the second period, the photoelectric conversionunits of the plurality of pixels accumulate charge generated in thesecond period.
 5. The image pickup apparatus according to claim 4,wherein at the third time point, the first transfer switches of theplurality of pixels transfer the charge generated by the photoelectricconversion units in the second period into the holding unit so that theholding units of the plurality of pixels hold the charge generated inthe first period and the charge generated in the second period.
 6. Theimage pickup apparatus according to claim 2, wherein at the second timepoint, the first transfer switch of the at least one of the plurality ofpixels is controlled from off to on.
 7. The image pickup apparatusaccording to claim 2, wherein in the second period, the first transferswitches of the plurality of pixels are kept on, and in the secondperiod, the holding units of the plurality of pixels hold chargegenerated by the photoelectric conversion units in the second period. 8.The image pickup apparatus according to claim 2, wherein through thesecond period within the first exposure period, the second transferswitches of the plurality of pixels are kept off, and through the secondexposure period, the second transfer switches of the plurality of pixelsare kept off.
 9. The image pickup apparatus according to claim 2,wherein each of the plurality of pixels includes a discharge switchconfigured to discharge charge of the photoelectric conversion unit, thedischarge switch of the at least one of the plurality of pixels is keptoff from the first time point to the second time point.
 10. The imagepickup apparatus according to claim 9, wherein the accumulation ofcharge is started by controlling the discharge switch from on to off.11. The image pickup apparatus according to claim 2, wherein theaccumulation of charge is started by controlling the first transferswitch from on to off.
 12. The image pickup apparatus according to claim2, wherein the holding unit includes a first semiconductor region of afirst conductive type which holds the charge and a second semiconductorregion of a second conductive type disposed on the first semiconductorregion.
 13. The image pickup apparatus according to claim 2, furthercomprising: waveguides disposed so as to correspond to the photoelectricconversion units of the plurality of pixels.
 14. The image pickupapparatus according to claim 2, further comprising: a semiconductorsubstrate including the photoelectric conversion units and the holdingunits disposed therein, wherein an area of an orthogonal projection ofthe photoelectric conversion unit onto a plane parallel to a surface ofthe semiconductor substrate is smaller than an area of an orthogonalprojection of the holding unit onto the plane.
 15. An image pickupsystem comprising: the image pickup apparatus set forth in claim 2; anda signal processing apparatus which processes a signal supplied from theimage pickup apparatus.
 16. The image pickup apparatus according toclaim 3, wherein in at least a portion of the second period, thephotoelectric conversion units of the plurality of pixels accumulatecharge generated in the second period.
 17. The image pickup apparatusaccording to claim 16, wherein at the third time point, the firsttransfer switches of the plurality of pixels transfer the chargegenerated by the photoelectric conversion units in the second periodinto the holding unit so that the holding units of the plurality ofpixels hold the charge generated in the first period and the chargegenerated in the second period.
 18. The image pickup apparatus accordingto claim 3, wherein at the second time point, the first transfer switchof the at least one of the plurality of pixels is controlled from off toon.
 19. The image pickup apparatus according to claim 3, wherein in thesecond period, the first transfer switches of the plurality of pixelsare kept on, and in the second period, the holding units of theplurality of pixels hold charge generated by the photoelectricconversion units in the second period.
 20. The image pickup apparatusaccording to claim 3, wherein through the second period within the firstexposure period, the second transfer switches of the plurality of pixelsare kept off, and through the second exposure period, the secondtransfer switches of the plurality of pixels are kept off.
 21. The imagepickup apparatus according to claim 3, wherein each of the plurality ofpixels includes a discharge switch configured to discharge charge of thephotoelectric conversion unit, the discharge switch of the at least oneof the plurality of pixels is kept off from the first time point to thesecond time point.
 22. The image pickup apparatus according to claim 21,wherein the accumulation of charge is started by controlling thedischarge switch from on to off.
 23. The image pickup apparatusaccording to claim 3, wherein the accumulation of charge is started bycontrolling the first transfer switch from on to off.
 24. The imagepickup apparatus according to claim 3, wherein the holding unit includesa first semiconductor region of a first conductive type which holds thecharge and a second semiconductor region of a second conductive typedisposed on the first semiconductor region.
 25. The image pickupapparatus according to claim 3, further comprising: waveguides disposedso as to correspond to the photoelectric conversion units of theplurality of pixels.
 26. The image pickup apparatus according to claim3, further comprising: a semiconductor substrate including thephotoelectric conversion units and the holding units disposed therein,wherein an area of an orthogonal projection of the photoelectricconversion unit onto a plane parallel to a surface of the semiconductorsubstrate is smaller than an area of an orthogonal projection of theholding unit onto the plane.
 27. An image pickup system comprising: theimage pickup apparatus set forth in claim 3; and a signal processingapparatus which processes a signal supplied from the image pickupapparatus.